The present invention relates to improvements in clamping screws or shear bolts used to exert a clamping force on an object or objects by the screwing and tightening of the bolt thereon.
As a conventional bolt is tightened to exert a clamping force on an object the torque required to further tighten the bolt increases. When the torque reaches a required level to provide the clamping force the conventional shear bolts are provided with reduced necks and shear at this point such that the head portion is removed. The shearing action of the bolt is commonly successful but the position of the shear is entirely dependent upon the positioning of the neck.
This causes problems in many uses and when the article upon which the bolts are used is an electrical cable connector and the objects to be clamped are cable cores, the shank of the bolt which remains can protrude above the surface of the connector. When a connector is used, portions of the two cable cores to be connected are placed in the connector and covered and shear bolts are applied to the connector to be screwed into threaded bores therein to provide a clamping force onto the cores and hold the same in place. When a certain clamping torque is reached the bolt shears at the neck and can leave part of the bolt shank protruding above the surface of the connector. The existence of these protrusions cause particular concern when used with high voltage cable connectors as each of the protrusions can cause what is known as a "corona" effect to occur. A corona effect occurs due to the attraction of high voltage stresses at points which are of a different form than the connector such as the protrusions caused by the sheared bolts. The corona effect leads to the insulation material around and adjacent the protrusions being eroded and destroyed by the high voltage stresses.
Further problems are caused when it is desired that the connector be coated by a heat shrinkable insulating material or placed in a close fitting sleeve of insulating material to protect the same once the cable cores are in position. The existence of the protrusions on the connector can produce protrusions on the insulating material which are undesirable or may prevent the fitting of the sleeve.
In an attempt to overcome these problems it is possible to use packing pieces in conjunction with the bolt when the cable core to be clamped is a relatively small diameter cable compared to the bore of the connector and/or to use differing lengths of bolts for each diameter of cable core to be clamped. These are not ideal solutions as the use of additional components adds time to the fitting of the bolts and the range of sizes of bolts which now exist is excessively large and expensive to stock.
The aim of the present invention is to provide a bolt or screw clamping means for use in clamping an object in position, said bolt being provided with means to allow at least a portion to shear off once a predesignated clamping torque is reached and to provide drive means for the same, the said apparatus of a form to allow the position and plane of the shear to be controlled and predicted thereby eliminating the production of protrusions on the surface of the article when the bolt shears.